Can you create a web folder  to the parent folder? If you can and you can see
the folder, try renaming the folder.

but I cant delete  or rename the folder. There is one document in
the folder  that is checked-out but I cannot manipulate this document either. I
can open it but I cannot check-in, publish, delete, rename, etc.

Try to write down a web part to delete  the folder. put the following code in the
web part.

dim sFolderUrl
dim objFolder
sFolderUrl = " http://computerName/Workspace/... (intranet Url)"
set objFolder = CreateObject("CDO.KnowledgeFolder")
objFolder.DataSource.Open sFolderUrl,,3
objFolder.Delete
set objFolder = nothing

and make sure you have coordinator/author role on that folder.
I didn't try it since I couldn't create a folder  with ":" in it.

Try to write down a web part to delete  the folder. put the following code in the
web part.

dim sFolderUrl
dim objFolder
sFolderUrl = " http://computerName/Workspace/... (intranet Url)"
set objFolder = CreateObject("CDO.KnowledgeFolder")
objFolder.DataSource.Open sFolderUrl,,3
objFolder.Delete
set objFolder = nothing

and make sure you have coordinator/author role on that folder.
I didn't try it since I couldn't create a folder  with ":" in it.

So I think possibly WSS stores the folder  in a different name. Try to output the
folder's "DAV:href" property by querying its parent folder's Subfolders. If WSS
stores that in a different name, then you probably could delete  it by using the
web part.
You should know how to list a folder's all subfolders. If you don't know, I can
write down some code for you.